1. Field of the Invention
The invention relates to an apparatus for liquid treatment of wafer-shaped articles, and a liquid control ring for use in such apparatus.
2. Description of Related Art
Liquid treatment includes both wet etching and wet cleaning, wherein the surface area of a wafer to be treated is wetted with a treatment liquid and a layer of the wafer is thereby removed or impurities are thereby carried off. A device for liquid treatment is described in U.S. Pat. No. 4,903,717. In this device the distribution of the liquid may be assisted by the rotational motion imparted to the wafer.
Single wafer wet processing of semiconductor wafers typically proceeds through a series of process modules, each of which contains a group of spin chucks such as that described in the above-referenced U.S. Patent. One typical process stage is referred to as “bevel etch”, and involves etching the back side of a silicon wafer as well as the outer periphery of the front or device side of the wafer. Etching of the device side is to a controlled extent of only a few millimeters at the outer periphery of the wafer.
A spin chuck adapted to perform bevel etching of semiconductor wafers is described in commonly-owned U.S. Pat. No. 7,172,674, and certain aspects of that spin chuck are illustrated in present FIGS. 1 and 2. As shown in FIG. 1, a chuck 11 supports a wafer W by gas flow in the direction of the arrows G, according to the Bernoulli principle. Thus, pins 53 restrain the wafer W laterally, whereas the wafer is held to the chuck by the counterbalancing pressures created by the gas flow beneath the wafer.
Chuck 11 is equipped with a ring 2 whose upper surface is spaced from the underside of wafer W by a defined small gap 15 of axial extent g. As shown in FIG. 2, as etching liquid L is dispensed onto the upwardly facing back side of wafer W, it also flows around the edge of wafer W and is drawn into gap 15 by capillary action. Ingress of etching liquid L halts at the radially inner edge of ring 15, where a meniscus is formed. The radial extent c of etching effected on the device or downwardly facing side of wafer W is therefore defined and limited by the extent of overlap b between wafer W and ring 15.
While such a device can form a highly accurate bevel edge, the process window within which optimum results are achieved is relatively narrow. Factors potentially affecting the quality and uniformity of the bevel etch, and its consistency from one wafer to the next, include the nature of the material to be removed from the wafer surface, the composition and physical properties of the etching liquid, the process temperature, the size of the gap 15, the diameter of the wafer being processed, and the speed of rotation of the chuck. For example, the capillary gap could be filled, such that the liquid stops there and does not flow inwardly. In such a case the liquid is not replaced or renewed during processing.
During single wafer edge etching processing, the etching area in practice is often not uniform at edge of wafer. Out of spec or off-center edge bead removal (EBR) can result in several different process-related issues; for example, if the EBR width is too narrow, edge grippers on process tools can touch the photoresist and cause it to crack and flake off, causing contamination. If the edge exclusion is too wide, there will be less surface area available for chip formation.
It would therefore be desirable to provide a method and apparatus for performing bevel etch with high accuracy and reproducibility over a broader process window, especially for relatively wider extents of edge etching. However, existing technology has limitations for controlling this range.